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The Global Positioning System provides the geocentric position for individual receivers or the relative position between co- observing receivers. To achieve accuracy, a computer routine package, GPS analysis Software at MIT (GAMIT),.is employed to evaluate the atmospheric total delay which is the sum of the atmospheric dry and wet delays. For given ground-based location, altitude, pressure, and temperature, an existing model accurately derives the atmospheric dry delay. Therefore, the atmospheric (GPS) wet delay can be obtained by subtracting the model day delay from the GPS total delay. Good agreement between wet delays obtained from radiosondes and GPS shows that the GPS wet delay can be used to study changes of the water vapor during various seasons, front passages, and Typhoon period.
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